The penetration resistance to rotating drill bits, measured, for example, via the power consumption of the driving electric motor with a known, preferably linear, characteristic curve, correlates with the density of the drilled material, provided that the geometry of the bit also satisfies specific conditions (inter alia, the tool tip is wider than the shank). This measurement principle has been applied since 1986 to wood, plastics, soils, sandstone and other materials. The drilling resistance profile achieved in this way generally contains information about the state of the material, from which conclusions can be drawn about its stability and loadbearing capacity and also about further properties (in the case of trees, about the annual ring growth, for example). Because of the normally a few millimeters thick bit, the significance is restricted to the local area, so that further measurements often have to be made at a certain spacing. At the bottoms of wooden masts and the tops of floor beams, for example, two measurements are usually made at a spacing of a few centimeters beside each other, in order to be able to detect internal damage with high reliability.
While the presence and extent of damage along the drilling distance can therefore as a rule be detected quite reliably, statements about possible adjacent damage and about the causes of damage are barely possible. The latter is in turn examined in many materials, in particular in biological ones like wood, via a measurement of the electrical conductivity, since the electrical conductivity depends on moisture content, the occurrence of fungus and on other contained substances, which can indicate the causes of damage.
The electrical conductivity, for example in wood, has been measured for decades by two electrically conductive pins being inserted and the electrical resistance between them being measured. It is influenced by water content and substances contained in the wood and by any fungus that may be present or other organisms that degrade wood which, inter alia, increase the number of ions in the wood.
From the combined, optional application of drilling resistance and conductivity measurements, in many cases conclusions can be drawn better as to the causes of damage that has been determined and is to be expected. However, this combination is also subject to some restrictions.
The penetration depth of drilling resistance measuring methods in trees and timbers reaches beyond 1 meter. The pins of electrical conductivity measurements, on the other hand, normally reach only a few to several centimeters. Therefore, the internal areas that are important to damage analysis, for example about 20 cm below soil level at the bottom of wooden masts, generally cannot be reached with the electrical resistance measurement. Although the known electrotomography of trees, the subject of a patent application in 1999 in combination with sound tomography, reaches greater penetration depths, in the case of installed timber is difficult to apply on account of the timbers seldom being accessible all around (similar to the case in wooden masts). In addition, the drying and shrinkage cracks that normally occur there massively restrict the significance.
Since the electrical conductivity likewise depends on the water content and on the presence of anions and cations, for example resulting from fungal infection, it is not possible to conclude from the conductivity values on their own that damaging degradation of wood is taking place. Therefore, the combination with the drilling resistance measurement is expedient since, given an appropriately good and linear technical resolution, it shows whether timber has already been degraded. However, such types of damage can be determined in this way only when timber degradation worth mentioning has occurred. To this extent, there can be combinations in which, even via a combination of drilling resistance measurement and electrical conductivity measurement, no clear statements about early stages of pathological fungal infection are possible (in particular not at greater drilling depths).
In addition, there are cases of a naturally increased but not pathogenic water content and non-damaging fungal infection—although both increase the conductivity appropriately considerably, there is no risk to the tree. Therefore, following conductivity measurements on the tree, erroneous assessments and felling often occurred, which in retrospect proved to be unnecessary.
However, with the water content, the thermal conductivity of the wood also increases in addition to the electrical conductivity. The measurement thereof therefore permits differentiation as to whether an increased electrical conductivity is brought about only by an increased water content or else by pathological infection. In the case of pathological infection, the electrical conductivity rises substantially more highly than only as a result of increased moisture.
An optionally combined application of the methods described here has barely been carried out hitherto for reasons of expenditure, time and costs and, on account of the given application restrictions in the case of the previously available technical devices (e.g. the low penetration depth of the electrical and thermal measurements), would provide no important progress corresponding to the effort.
For the most error-free detection even of early stages of damage deep in the interior of trees and timbers, including wooden masts, there is therefore a need for a combination of the positive properties of the previously separate methods described here and overcoming their restrictions.